Electromotive drive system for engine-driven vehicle

ABSTRACT

An electromotive drive system including a small motor and a small-capacity battery more effectively improves the fuel economy of an engine-driven vehicle. An electromotive drive system includes: a motor for driving a vehicle; a battery storing electrical energy to rotate the motor; an inverter; a converting mechanism transmitting rotation of the motor to a drive shaft at a predetermined conversion ratio independently of a conversion ratio at which an engine is driven; and a control unit controlling an operation of the inverter. The control unit is configured so that upper vehicle speed limits, to which the motor is allowed to operate, can be set separately during powering and during regeneration, respectively.

RELATED APPLICATIONS

This application is the Continuation Application of U.S. patentapplication Ser. No. 14/764,147, filed on Jul. 28, 2015, which is theU.S. National Phase under 35 U.S.C. § 371 of International PatentApplication No. PCT/JP2014/001145, filed on Mar. 3, 2014, which in turnclaims the benefit of Japanese Application No. 2013-077110, filed onApr. 2, 2013, the disclosures of which Applications are incorporated byreference herein.

TECHNICAL FIELD

The present invention relates to an electromotive drive device includinga motor and a battery, and more particularly relates to an electromotivedrive device for use in an engine-driven vehicle.

BACKGROUND ART

In recent years, so-called “hybrid vehicles”, each including an engineand a motor, have been popularized. Motors have advantages over engines,because motors achieve faster torque response, higher torquecontrollability, higher energy conversion efficiency, and less noise andvibration than engines. On the other hand, engines have their ownadvantages, because engines use petroleum fuel with high energy densityand achieve a long cruising distance. Various techniques for hybridvehicles have been and are being developed to take full advantage of anengine and a motor that are used in combination as a hybrid powersource.

There are various types of hybrid vehicles, examples of which includeseries, parallel, and series-parallel hybrid vehicles, which areclassified according to the method of coupling between a motor and anengine. A series hybrid vehicle is driven by a drive motor with anengine mechanically disengaged from wheels. Power is transmitted inseries from the engine to a power generation motor, to a secondarybattery, to the drive motor, and then to the wheels in this order. Aparallel hybrid vehicle is driven by mechanically connecting a drivemotor and an engine to wheels. Power is transmitted in parallel from theengine to a transmission and then to the wheels and from a secondarybattery to the drive motor and then to the wheels. A series-parallelhybrid vehicle is a combination of the series hybrid type and theparallel hybrid type. Examples of series-parallel hybrid vehiclesinclude a type in which power is changed by engagement and disengagementof a clutch and a power-split type in which the power distribution ratiois changed using a planetary gear.

Patent Document 1 discloses a parallel hybrid vehicle which includes anengine driving either one pair of wheels (namely, front wheels or rearwheels) and a motor driving the other pair, and in which a motor speedreduction gear transmitting the driving force of the motor to the wheelsincludes a switching mechanism engaging and disengaging a sleevewith/from a clutch gear.

CITATION LIST Patent Document

PATENT DOCUMENT 1: Japanese Unexamined Patent Publication No.2011-255763

SUMMARY OF THE INVENTION Technical Problem

In the case of a parallel hybrid vehicle where power from an engine andpower from a motor are transmitted in parallel to wheels, ahigh-capacity secondary battery is required to secure electric power fordriving the motor. This has not only led to an increase in cost but alsocaused the problem of charge/discharge losses. Since there is a need tofrequently operate the drive motor in a power generation mode in orderto charge the battery, fuel economy has not always been sufficientlyimproved.

A configuration in which a non-drive shaft of an engine-driven vehicleis retrofitted with an electromotive assist drive device including amotor has also recently been provided. This electromotive drive deviceprovides appropriate assistance in a situation where an engine cannothelp resorting to a region having low thermal efficiency, e.g., when thevehicle has just started and is accelerating or when the vehicle isbeing driven at low speeds. That is to say, the motor output does notneed to be so high, because assistance is intended to be provided to thepoint that motor-driven wheels help move a vehicle always travelling onengine-driven wheels. However, the present inventors discovered viaexperiments that fuel economy cannot always be sufficiently improved bysimply using motor drive in such a configuration while the vehicle thathas just started is accelerating or while the vehicle is being driven atlow speeds.

It is an object of the present invention to improve the fuel economy ofan engine-driven vehicle more effectively using an electromotive drivedevice including a small motor and a low-capacity battery.

Solution to the Problem

According to an aspect of the present invention, an electromotive drivedevice for use in an engine-driven vehicle includes: a motor for drivingthe vehicle; a battery storing electrical energy to rotate the motor; aninverter transforming power output from the battery into alternatingcurrent to supply the alternating current to the motor, and alsotransforming power regenerated by the motor into direct current tosupply the direct current back to the battery; a control unitcontrolling an operation of the inverter; and a converting mechanismtransmitting rotation of the motor to a drive shaft of the vehicle at apredetermined conversion ratio independently of a conversion ratio atwhich an engine is driven. The control unit is configured to set anupper vehicle speed limit, to which the motor is allowed to operate, tobe higher during regeneration than during powering and/or to limit amaximum driving force of the motor more strictly during powering thanduring regeneration.

Optionally, the control unit may be configured to enable dynamicadjustment of at least one of the upper vehicle speed limit, to whichthe motor is allowed to operate, or the maximum driving force of themotor during powering and/or during regeneration.

Furthermore, the control unit may dynamically adjust at least one of theupper vehicle speed limit, to which the motor is allowed to operate, orthe maximum driving force of the motor during powering and/or duringregeneration in accordance with a piece of information on a state ofcharge (SOC) of the battery.

According to another aspect of the present invention, an electromotivedrive device for use in an engine-driven vehicle includes: a motor fordriving the vehicle; a battery storing electrical energy to rotate themotor; an inverter transforming power output from the battery intoalternating current to supply the alternating current to the motor, andtransforming power regenerated by the motor into direct current tosupply the direct current back to the battery; and a convertingmechanism transmitting rotation of the motor to a drive shaft of thevehicle at a predetermined conversion ratio independently of aconversion ratio at which an engine is driven. The predeterminedconversion ratio is set so that if a rotational speed of the motor ismaximum, the vehicle has a predetermined speed. The motor has a ratedoutput of less than or equal to 15 kW. The battery has a rated capacityof less than or equal to 500 Wh. The predetermined speed is higher than60 km/h and lower than 100 km/h.

In one embodiment, the predetermined speed may be 80 km/h.

Alternatively, the predetermined speed may be a half or less as high asa maximum speed of the engine-driven vehicle.

Advantages of the Invention

According to the present invention, the fuel economy of a vehicle isimproved more effectively by using an electromotive drive deviceincluding a small motor and a low-capacity battery in an engine-drivenvehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A diagram illustrating a general configuration for anelectromotive drive device according to an embodiment.

FIG. 2A graph showing exemplary motor characteristics in anelectromotive drive device according to the embodiment.

FIG. 3A graph showing how fuel economy varies as powering and/orregeneration condition(s) is/are changed.

FIG. 4A graph showing how the state of charge (SOC) of a battery variesas powering condition is changed.

FIG. 5A conceptual diagram showing motor characteristics of anelectromotive drive device according to an embodiment.

FIG. 6A schematic diagram showing the flow of electromechanical energyfrom tires to a battery in an electromotive drive device according to anembodiment.

FIG. 7A conceptual diagram showing other exemplary motor characteristicsof an electromotive drive device according to an embodiment.

FIG. 8 An exemplary control flow during powering.

FIG. 9 An exemplary control flow during regeneration.

FIG. 10 Shows how the upper speed limit may be controlled duringpowering and regeneration according to the SOC of a battery.

FIG. 11 Another exemplary configuration in which an electromotive drivedevice according to an embodiment is used.

FIG. 12 Still another exemplary configuration in which an electromotivedrive device according to an embodiment is used.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings.

FIG. 1 is a diagram showing a general configuration for an electromotivedrive device according to an embodiment. The electromotive drive deviceaccording to this embodiment is for use in an engine-driven vehicle. InFIG. 1, a vehicle 1 is supposed to be a front-engine, front-wheel-drive(FF) vehicle, and an electromotive drive device 10 according to thisembodiment is provided for rear wheels. Front wheels are driven by anengine 4. The electromotive drive device 10 includes a motor 11 fordriving an axle 2 for the rear wheels, a battery 12 storing electricalenergy to rotate the motor 11, and an inverter 13 transforming poweroutput from the battery 12 into alternating current to supply it to themotor 11 and also transforming power regenerated by the motor 11 intodirect current to supply it back to the battery 12. The electromotivedrive device 10 is supposed to operate at 48 V, for example. The battery12 may be a 48V lithium ion battery. However, this is only an example ofthe present invention. The electrical energy stored in the battery 12 isalso supplied to a load other than the motor 11 in the vehicle 1.

The electromotive drive device 10 further includes a differential gear14, an electronic control unit (ECU) 15, and a direct current to directcurrent (DC/DC) converter 16. The differential gear 14 functions as aconverting mechanism that transmits the rotation of the motor 11 to theaxle 2 at a predetermined conversion ratio. In other words, power istransmitted from the motor 11 to the axle 2 at a conversion ratioindependently of the conversion ratio at which the engine is driven. TheECU 15 receives pieces of information on the vehicle speed and the stateof charge (SOC) of the battery 12 and any other piece of information,and transmits a control signal to the inverter 13 based on these piecesof information. The ECU 15 operates in cooperation with another ECU 3provided for the engine. The inverter 13 controls the operation of themotor 11 in accordance with the control signal supplied from the ECU 15.The DC/DC converter 16 performs voltage transformation to lower 48 Vthat is the output voltage of the battery 12 to 12 V, for example. Notethat the electromotive drive device 10 does not always include the DC/DCconverter 16 as an essential element.

Note that in this example, the motor 11 is supposed to be a synchronousmotor including a permanent magnet and a small motor having a ratedoutput of 6 kW and a maximum output of about 8 kW and that the battery12 has a capacity of about 0.24 kWh. The output of the motor used inthis example is about one-tenth of that of a motor for use in afull-scale hybrid vehicle, and the capacity of the battery used in thisexample is also about one-tenth of that of a battery of such a vehicle.

Moreover, the differential gear 14 includes a clutch. The operation ofthis clutch is controlled in accordance with the control signal from theECU 15, and the motor 11 and the axle 2 are separated from each other bydisengaging the clutch. This is done in order to prevent the reversevoltage of the motor 11 that is a synchronous motor from becomingexcessively high in a high-speed range. Note that if the motor 11 is aninduction motor, the clutch is not always required.

FIG. 2 is a graph showing exemplary motor characteristics in anelectromotive drive device according to this embodiment. In FIG. 2, theordinate represents the driving force [N], and the abscissa representsthe vehicle speed [km/h]. In FIG. 2, the hatched region shows thecharacteristics of the motor 11 according to this embodiment. FIG. 2further shows the characteristics of the engine for respective gearratios (first, second, third, fourth, and fifth gear ratios). If thedriving force is positive, the vehicle is performing powering (i.e., thevehicle is being driven and accelerated). On the other hand, if thedriving force is negative, the vehicle is performing regeneration (i.e.,the vehicle is being decelerated with the engine brake).

Note that since the vehicle speed and the rotational speed of the motor11 correspond to each other, the hatched region corresponds to the N-Tcharacteristics of the motor 11 (the relationship between the rotationalspeed and load torque thereof).

As shown in FIG. 2, in this embodiment, the range where the motor 11 isallowed to operate is defined such that the vehicle speed is lower thanor equal to 80 km/h. In other words, the conversion ratio of thedifferential gear 14 is determined such that when the rotational speedof the motor 11 is maximum, the vehicle speed is 80 km/h, whichcorresponds to the maximum speed in JC08 mode. In this embodiment, thedriving force of the motor 11 is utilized in the speed range for urbanarea driving.

Here, it will be briefly described exactly how such a hybrid systemimproves the fuel economy. The effect of improving the fuel economyachieved by a hybrid system results mainly from a) high-efficiency pointtracking control of an engine and b) utilization of regenerative energy.In a hybrid system in which two motors are utilized, a combination ofthe actions a) and b) improves the fuel economy. In a hybrid system inwhich one motor is used, a combination of idle reduction and the actionb) improves the fuel economy in most cases. In recent years, the effectresulting from the action a) has been achieved significantly byinnovation and improvement in engine technologies. Also, if an attemptis made to obtain the effect resulting from the action a) using anelectromotive drive device, a large motor or a power generator will beneeded. Thus, it is recommended that in an electromotive drive deviceincluding a small motor, the fuel economy be improved with emphasis onthe action b), i.e., utilization of regenerative energy.

If a small motor having a power of about 8 kW were used continuouslyuntil the vehicle driven by the engine almost reaches its maximum speed,the torque would be insufficient in a normal range, and no effect wouldbe achieved. In addition, since the thermal efficiency of an engine issufficiently high in a high-speed range, a hybrid effect is difficult toobtain. On the other hand, to achieve an assistance effect while thevehicle that has just started is accelerating, the range in which asmall motor is used may be limited to, for example, 30 km/h or less.However, the studies and simulation carried out by the present inventorsalmost clearly revealed that this method would not always produce theeffect of reducing the fuel consumption to a satisfactory degree.

Thus, in this embodiment, the range in which the motor 11 is allowed tooperate is set to be 80 km/h or less, which is approximately as high asthe upper speed limit during normal driving. This thus ensures low-speedtorque (driving force) even with a small motor, and effectively improvesthe fuel economy.

In this example, the upper speed limit in the range in which the motor11 is allowed to operate is supposed to be 80 km/h. However, this isonly an example of the present invention, and the speed limit just needsto be determined so that the motor 11 is used in the normal range, andnot used in the high speed range. For example, the conversion ratio ofthe converting mechanism is set such that under the condition that therated output of the motor is 15 kW or less and the rated capacity of thebattery is 500 Wh or less, the upper speed limit in the range in whichthe motor 11 is allowed to operate is higher than 60 km/h and lower than100 km/h. According to such settings, the vehicle's fuel economy will beimproved even more effectively by providing an electromotive drivedevice including a small motor and a low-capacity battery for anengine-driven vehicle.

Alternatively, the upper speed limit in the range in which the motor 11is allowed to operate may be set to be higher than 60 km/h and a half orless as high as the maximum speed of an engine-driven vehicle, forexample.

Furthermore, in this embodiment, two upper speed limits, to which themotor 11 is allowed to operate, are supposed to be set separately duringpowering and during regeneration, respectively, in the operable rangedefined by the conversion ratio. This setting is done by the ECU 15.

FIG. 3 is a graph showing how the fuel economy varies as the poweringand/or regeneration condition(s) is/are changed. The abscissa representsthe upper speed limit [km/h] during powering, the ordinate representsthe fuel economy [km/l], and data obtained by changing the upper speedlimit during regeneration into 40 [km/h], 60 [km/h], and 80 [km/h] areplotted. FIG. 4 is a graph showing how the state of charge (SOC) of abattery varies as powering conditions are changed. The abscissarepresents the time [s], the ordinate represents SOC [%], and dataobtained by changing the upper speed limit during powering into 20[km/h], 40 [km/h], and 80 [km/h] are plotted. The vehicle speeds atrespective points in time are also plotted. FIGS. 3 and 4 show theresults of simulations carried out by the present inventors, the motoroutput is 8 kW, the battery capacity is 250 Wh, and the driving patterncorresponds to JC08 mode. The fuel economy of an engine-driven vehicle(with an idle reduction system) used as a base is 25.34 km/l.

As can be seen from FIG. 3, as the upper speed limit during regenerationrises, the amount of energy that can be extracted increases, and thefuel economy thus tends to be improved. Specifically, as the upper speedlimit during regeneration is increased from 40 km/h to 60 km/h and thento 80 km/h, the fuel economy is improved. On the other hand, it is notalways true that the upper speed limit during powering is preferablyhigh in terms of the fuel economy. Specifically, if the upper speedlimit during regeneration is 40 km/h, maximum fuel economy is achievedwhen the upper speed limit during powering is about 30 km/h. However, ifthe upper speed limit during regeneration is 60 or 80 km/h, maximum fueleconomy is achieved when the upper speed limit during powering is about40 km/h.

Such relationship between the upper speed limit during powering and thefuel economy is associated with the SOC of the battery. Specifically, ascan be seen from FIG. 4, if the upper speed limit during powering is toohigh, such as 80 km/h, the SOC of the battery soon decreases to thevicinity of a lower prohibited region, and a sufficient motor assistanceeffect cannot be achieved. Thus, the fuel economy rather declines. Ascan be seen from the foregoing description, the upper speed limit duringpowering has an appropriate range that does not include any excessivelyhigh or excessively low value, in terms of the SOC of the battery. Forexample, the graph in FIG. 3 shows that when the upper speed limitduring regeneration is 80 km/h and the upper speed limit during poweringis 36 km/h, the maximum fuel economy is achieved.

Thus, it can be said that to further improve the fuel economy, two upperspeed limits, to which the motor 11 is allowed to operate, arepreferably set separately during powering and during regeneration,respectively. For example, the upper speed limit during regeneration maybe set to be in the vicinity of the upper speed limit in the range inwhich the motor 11 is allowed to operate and which is determined by theconversion ratio, and the upper speed limit during powering may be setto be lower than the upper speed limit during regeneration and to be aspeed at which the vehicle frequently travels during urban area driving.By adopting such settings, use of the electromotive drive deviceaccording to this embodiment improves fuel economy even moreeffectively.

FIG. 5 is a conceptual diagram showing exemplary motor characteristicsof an electromotive drive device according to the embodiment. In FIG. 5,the upper speed limit, to which the motor 11 is allowed to operate, isset to be the speed “a” during powering and the speed “b” duringregeneration, and the speed a is lower than the speed b. The speed “a”is, for example, 40 km/h, and the speed “b” is, for example, 80 km/h.Note that it is effective to set separately the upper speed limits, towhich the motor is allowed to operate, during powering and duringregeneration, respectively, not only under the conditions described forthis embodiment, such as the range where the motor 11 is allowed tooperate, the output of the motor 11, and the capacity of the battery 12,but also under other conditions.

In the example shown in FIG. 5, the ECU 15 stops driving a vehicle usingthe motor 11 when the vehicle speed reaches the preset upper speed limit“a” (40 km/h) during powering. In other words, the motor 11 is turnedOFF at the upper speed limit “a” lower than the upper speed limit “b”during regeneration. The reason for this is as follows.

FIG. 6 is a schematic diagram showing the flow of electromechanicalenergy in an electromotive drive device according to the embodiment. Asshown in FIG. 6, during regeneration, as mechanical energy (kineticenergy) from tires 19 is transmitted to gears 18 including adifferential gear 14, to a motor 11, to an inverter 13, and then to abattery 12 in this order, the energy decreases by the losses caused bythese members. In this embodiment, the overall efficiency of such amechanical system and such an electrical system is, for example, 70%.The electric power stored in the battery 12 by regeneration istransmitted to the tires 19 in reverse direction during powering. Insuch a situation, the overall efficiency of the electrical andmechanical systems is also 70%, for example. Thus, 49% of the energyobtained by regeneration is transmitted to the tires 19 during powering.Therefore, in this embodiment, the upper speed limit “a” during poweringis independently set to be about half of the upper speed limit “b”during regeneration in consideration of possible losses, thusaccomplishing energy balance. Such an operation will enhance efficiencysignificantly.

In FIG. 5, two upper vehicle speed limits, to which the motor 11 isallowed to operate, are set separately during powering and duringregeneration, respectively. However, values to be set separately duringpowering and during regeneration are not only the vehicle speeds. Forexample, the maximum driving forces (absolute values) of the motor 11may also be set separately during powering and during regeneration.

FIG. 7 is a conceptual diagram showing other exemplary motorcharacteristics of an electromotive drive device according to theembodiment. In FIG. 7, the maximum driving force of the motor 11 is setto be the driving force A during regeneration, and is limited to thedriving force B during powering, and the driving force A is greater thanthe driving force B. The reason why such a limitation is imposed is thatenergy balance is to be accomplished as described with reference to FIG.6. Such an operation will also enhance efficiency significantly.

FIG. 5 shows an example in which two upper vehicle speed limits, towhich the motor 11 is allowed to operate, are set separately duringpowering and during regeneration, respectively. FIG. 7 shows an examplein which the maximum driving forces of the motor 11 are set separatelyduring powering and during regeneration. Optionally, both the uppervehicle speed limits, to which the motor 11 is allowed to operate, andthe maximum driving forces of the motor 11 may be set separately duringpowering and during regeneration. Even so, energy balance is alsoaccomplished as described above, thus achieving high efficiency.

FIG. 8 is an exemplary control flow during powering, and FIG. 9 is anexemplary control flow during regeneration. While the control flow ineach of FIGS. 8 and 9 is basically carried out by an ECU 15, control onthe engine is performed by the other ECU 3 provided for the engine.

The control to be performed during powering will be described withreference to FIG. 8. The driver of the vehicle 1 presses theaccelerator, and releases the brake (S11). Then, the ECU 15 receives apiece of information on the vehicle speed, and compares the currentspeed S of the vehicle 1 to the upper speed limit “a” for the motor 11during powering (S12). If the speed S is greater than the upper speedlimit “a”, the vehicle is not driven by the motor 11, and the ECU 15thus disengages the clutch of the differential gear 14, thereby settingthe torque of the motor 11 to be zero (S13). Then, the ECU 15 determinesthe throttle opening position or a power command for the engine, andnotifies the ECU 3 on the engine side of these determined values (S14).

On the other hand, if the speed S is lower than or equal to the upperspeed limit “a”, the ECU 15 determines the output Pref required for themotor based on the accelerator position (S15). Here, the calculation isperformed based on a predetermined map. Then, the speed S is convertedinto the rotational speed of the motor 11 using the conversion ratio ofthe differential gear 14, and a torque command Tref is determined basedon this rotational speed and the output Pref required for the motor(S16).

Then, the ECU 15 compares this torque command Tref to the maximum motortorque Tmm (S17). If the torque command Tref is less than or equal tothe maximum value Tmm, the vehicle is driven by the motor 11 inaccordance with this torque command Tref, and therefore, the ECU 15sends the ECU 3 an engine stop command (S18). On the other hand, if thetorque command Tref is greater than the maximum value Tmm, the ECU 15sets the output of the motor 11 to be maximum (S19). Then, the ECU 15subtracts the output of the motor 11 to determine the throttle openingposition (S20).

Next, a control to be performed during regeneration will be describedwith reference to FIG. 9. The driver of the vehicle 1 presses the brake,and releases the accelerator (S21). Then, the ECU 15 receives a piece ofinformation on the vehicle speed, and compares the current speed S ofthe vehicle 1 to the upper speed limit “b” for the motor 11 duringregeneration (S22). If the speed S is greater than the upper speed limit“b”, no regenerative operation is performed by the motor 11, and the ECU15 thus disengages the clutch of the differential gear 14, therebysetting the torque of the motor 11 to be zero (S23).

On the other hand, if the speed S is lower than or equal to the upperspeed limit “b”, the ECU 15 receives a piece of information on the brakepedal force, and determines a regenerative torque Tref based on thispiece of information (S24). Examples of pieces of information on thebrake pedal force include a brake oil pressure, a brake stroke, and a Gsensor value of the brake. Here, the calculation is performed based on apredetermined map. Then, the speed S is converted into the rotationalspeed of the motor 11 using the conversion ratio of the differentialgear, and the maximum regenerative torque Trm at this rotational speedis determined (S25).

Then, the ECU 15 compares the regenerative torque Tref to the maximumregenerative torque Trm (S26). If the regenerative torque Tref isgreater than the maximum regenerative torque Trm, the ECU 3 determinesthe brake oil pressure by subtracting a value corresponding to themaximum regenerative torque Trm (S27). On the other hand, if theregenerative torque Tref is less than or equal to the maximumregenerative torque Trm, the motor 11 produces a regenerative output ofthe maximum regenerative torque Trm.

Furthermore, the upper vehicle speed limit, to which the motor 11 isallowed to operate, may be dynamically changed in accordance with apredetermined condition. In that case, the upper vehicle speed limit maybe changed both during powering and during regeneration, or eitherduring powering or during regeneration.

Next, it will be described how the upper speed limit, to which the motor11 is allowed to operate, may be dynamically changed according to theSOC of the battery 12. FIG. 10 shows an exemplary control of the upperspeed limit according to the SOC. In the example shown in FIG. 10, ifthe SOC of the battery 12 is in normal state, the upper speed limit “a”for the motor 11 during powering is set to be 40 km/h, and the upperspeed limit “b” during regeneration is set to be 80 km/h. The ECU 15 hasreceived a piece of information on the SOC of the battery 12, and if theSOC increases to exceed 70%, the upper speed limit “a” during poweringis gradually increased. Thus, the electrical energy stored in thebattery 12 is further consumed, thereby reducing an increase in SOC.Furthermore, once the SOC exceeds 70%, the upper speed limit b duringregeneration will be gradually decreased. Thus, the amount of electricalenergy stored in the battery 12 will decrease, too. Such a dynamicchange of the upper speed limit prevents the SOC of the battery 12 fromincreasing excessively. Naturally, the upper speed limit may be changedonly during powering or during regeneration, and the SOC range in whichthe upper speed limit is to be changed, how to change the upper speedlimit, and other conditions are not limited to those shown here.

Naturally, even if the SOC of the battery 12 has decreased, the same orsimilar control may be performed. For example, if the SOC is below 30%,a control may be performed by decreasing the upper speed limit “a”during powering or by increasing the upper speed limit “b” duringregeneration.

Optionally, the torque command may be controlled according to the SOC ofthe battery 12. For example, after having determined the torque commandTref in Step S16 of the control flow during powering in FIG. 8, the ECU15 receives a piece of information on the SOC of the battery 12. If theSOC is less than 40%, for example, the torque command Tref is multipliedby a coefficient of less than one, which is determined based on the SOC.In this manner, an excessive decrease in SOC may be regulated. Likewise,after the regenerative torque Tref has been determined in step S24 ofthe control flow during regeneration in FIG. 9, the ECU 15 receives apiece of information on the SOC of the battery 12. If the SOC is greaterthan 70%, for example, the regenerative torque Tref is multiplied by acoefficient of less than one, which is determined based on the SOC.Thus, an excessive increase in SOC may be regulated.

Note that the piece of SOC information used above may be estimated basedon, for example, the amounts of charge and discharge current withrespect to the battery 12.

Optionally, in such a configuration in which the maximum driving forcesof the motor 11 are set separately during powering and duringregeneration, respectively, as shown in FIG. 7, the maximum drivingforce of the motor 11 may be dynamically adjusted in accordance with,for example, a piece of information on the SOC of the battery 12 as inthe dynamic adjustment of the upper speed limit just described above. Inthis case, the maximum driving force may also be changed both duringpowering and during regeneration, or only during powering or duringregeneration. Furthermore, in a configuration in which both the upperspeed limit, to which the motor 11 is allowed to operate, and themaximum driving force of the motor 11 are set separately during poweringand during regeneration, both or either of the upper speed limit and themaximum driving force may be dynamically adjusted. These configurationsalso provide the same or similar advantages as in a situation where theupper speed limit is dynamically adjusted as described above.

In the embodiment described above, the electromotive drive deviceaccording to this embodiment is supposed to be provided for the rearwheels of the vehicle 1 that is an FF vehicle. However, this is only anexemplary application of the electromotive drive device according tothis embodiment. For example, the electromotive drive device accordingto this embodiment may be provided for the front wheels of afront-engine, rear-wheel-drive (FR) vehicle, or may be attached to thedriving wheels of an engine.

FIG. 11 shows another exemplary application of an electromotive drivedevice according to this embodiment. In the configuration shown in FIG.11, an electromotive drive device 20 according to this embodiment isattached to an axle 6 driven by an engine 5. The electromotive drivedevice 20 includes a motor 21 for driving the axle 6, a battery 22storing electrical energy to rotate the motor 21, and an inverter 23transforming power output from the battery 22 into alternating currentto supply it to the motor 21, and also transforming power regenerated bythe motor 21 into direct current to supply it back to the battery 22.The electromotive drive device 20 further includes a transmission 24 andan ECU 25. The transmission 24 functions as a converting mechanism thattransmits the rotation of the motor 21 to the axle 6 at a predeterminedconversion ratio independently of the conversion ratio at which theengine 5 is driven. The ECU 25 receives pieces of information on thevehicle speed and the SOC of the battery 22 and other pieces ofinformation, and supplies a control signal to the inverter 23 based onthese pieces of information. The inverter 23 controls the operation ofthe motor 21 in accordance with the control signal supplied from the ECU25.

FIG. 12 shows still another exemplary application of an electromotivedrive device according to this embodiment. In the configuration in FIG.12, the vehicle 1 includes a propeller shaft 31, and torque transmittingmeans 32 operating based on the difference in rotational speed betweenfront and rear wheels. The propeller shaft 31 and the torquetransmitting means 32 are mechanically connected to a differential gear14 provided for the rear wheels so that all of the wheels are driven byan engine 4. Specifically, the output of the engine 4 is divided intotwo outputs, one of which is input to a differential gear 33 for thefront wheels, and a drive shaft 6 for the front wheels is directlydriven by the engine. On the other hand, the other output of the engine4 is input to the propeller shaft 31, and is transmitted through thetorque transmitting means 32 to the differential gear 14 for the rearwheels. Thus, the drive shaft 2 for the rear wheels is not directlydriven by the engine. Note that a motor 11 is also mechanicallyconnected through the differential gear 14 and a clutch 35 to the driveshaft 2 for the rear wheels in the same or similar way. Thus, the motor11 is configured to be substantially directly connected to the driveshaft 2 for the rear wheels and connected through the torquetransmitting means 32 to the engine 4. In other words, the differentialgear 14 transmits the rotation of the motor 11 to the drive shaft 2 thatis not directly driven by the engine in the vehicle 1, and the output ofthe engine 4 is transmitted through the propeller shaft 31 and thetorque transmitting means 32 to the differential gear 14. The electricalconnection of the motor 11 may be identical or similar to the one shownin FIG. 1, and will not be described in detail.

Some characteristic operation of the configuration shown in FIG. 12 willbe described. In the electromotive drive device 10 shown in FIG. 12, anECU 15 performs a control so that when the vehicle 1 starts moving, therotational speeds of the motor 11 and the engine 4 driven are in syncwith each other. This control makes the torque input from the propellershaft 31 to the torque transmitting means 32 substantially equal to thetorque input from the motor 11 thereto within tolerance, and therefore,the torque from the engine 4 is hardly transmitted to the drive shaft 2.This prevents the drive shaft 2 from being unnecessarily driven when thevehicle starts moving on an ordinary road that is not especiallyslippery, thus reducing the load on the engine 4. As a result, the fueleconomy of the vehicle 1 is improved.

Even if when the vehicle starts moving on a slippery road, the wheelsengaged with the drive shaft 6 provided for the engine 4 slip, thewheels engaged with the drive shaft 2 driven by the motor 11 can stilldrive the vehicle 1. This thus improves fuel economy when the vehiclestarts moving, irrespective of the road surface condition. If after thevehicle has started moving, the motor 11 stops being driven at the upperspeed limit “a” during powering as shown in, for example, FIG. 5, thevehicle may be driven by mechanical four-wheel drive using only theengine 4 from then on.

Next, the operation of the vehicle 1 climbing a snowy slope will bedescribed as another characteristic operation thereof. On sensing thatthe front wheels have slipped based on the difference in rotationalspeed between the front and rear wheels of the vehicle 1, the ECU 15performs a control so that the direction of the force of the motor 11becomes intentionally opposite from that of rotation of the propellershaft 31. This causes the torque transmitting means 32 to significantlyslip, and the torque of the engine 4 is actively transmitted to thedrive shaft 2 for the rear wheels. As a result, the vehicle achieveshigh ground covering ability without making any significant slip evenwhen climbing a snowy slope.

In a general all-wheel-drive vehicle including torque transmitting means32 that utilizes the shearing resistance of high-viscosity oil, torqueis transmitted based on the difference in rotational speed between itsfront and rear wheels. Thus, while the front wheels significantly slip,the torque of the front wheels is transmitted to the rear wheels, thussometimes making the vehicle stability insufficient.

Meanwhile, in a type of a vehicle in which rear wheels are driven simplyby only a motor 11 without using any mechanical connection to an engine4, high torque is required at low speeds to ensure a high driving forcerequired in climbing a slope. For this reason, to improve fuel economyby using regenerative energy at intermediate and high speeds and achievegradeability at the same time, a large battery and a large motor may beused, or a motor may be further provided with a two- or three-speedtransmission. Such components will all cause a significant increase incost.

On the other hand, the configuration shown in FIG. 12 solves all ofthese problems. Specifically, the configuration shown in FIG. 12provides a low-cost vehicle 1 having all of high stability, high groundcovering ability, and good fuel economy by adding a small motor 11 to anall-wheel-drive vehicle including existent torque transmitting means 32.

An electromotive drive device according to this embodiment is alsoapplicable to an in-wheel drive vehicle, for example.

DESCRIPTION OF REFERENCE CHARACTERS

1 Vehicle

2, 6 Drive Shaft

4, 5 Engine

10 Electromotive Drive Device

11 Motor

12 Battery

13 Inverter

14 Differential Gear (Converting Mechanism)

15 ECU (Control Unit)

20 Electromotive Drive Device

21 Motor

22 Battery

23 Inverter

24 Transmission (Converting Mechanism)

25 ECU (Control Unit)

31 Propeller Shaft

32 Torque Transmitting Means

The invention claimed is:
 1. An electromotive drive system for anengine-driven vehicle, the system comprising: an electricity drivenmotor that, at a point in time, either operates in a drive mode or apower generation mode, wherein, in the drive mode, the electricitydriven motor drives the vehicle, and in the power generation mode, theelectricity driven motor generates electric power; a battery that storeselectrical energy that is used to rotate the electricity driven motor;an inverter that transforms electrical energy that is output from thebattery into an alternating current to supply the alternating current tothe electricity driven motor, and transforms electric power regeneratedby the electricity driven motor into a direct current to supply thedirect current back to the battery; a converter that transmits, to adrive shaft of the vehicle, a rotation of the electricity driven motorat a first conversion ratio independently of a second conversion ratioat which a gas powered engine is driven, the first conversion ratiobeing predetermined; and an electronic controller that controls anoperation of the inverter, sets a first maximum driving force of theelectricity driven motor, the first maximum driving force being amaximum driving force of the electricity driven motor when theelectricity driven motor is operating in the drive mode, and the firstmaximum driving force being in an absolute value, sets a second maximumdriving force of the electricity driven motor, the second maximumdriving force being a maximum driving force of the electricity drivenmotor when the electricity driven motor is operating in the powergeneration mode, and the second maximum driving force being in anabsolute value, wherein the first maximum driving force being smallerthan the second maximum driving force.
 2. The electromotive drive systemof claim 1, wherein the electronic controller dynamically adjusts thefirst maximum driving force of the motor and the second maximum drivingforce of the electricity driven motor.
 3. The electromotive drive systemof claim 2, wherein the electronic controller dynamically adjusts thefirst maximum driving force of the electricity driven motor and thesecond maximum driving force of the electricity driven motor inaccordance with a piece of information on a state of charge (SOC) of thebattery.
 4. The electromotive drive system of claim 1, wherein: thefirst conversion ratio is preset so that when a rotational speed of theelectricity driven motor is at a maximum, the vehicle is set to travelat a predetermined speed, the electricity driven motor has a ratedoutput of less than or equal to 15 kW, the battery has a rated capacityof less than or equal to 500 Wh, and the predetermined speed is higherthan 60 km/h and lower than 100 km/h.
 5. The electromotive drive systemof claim 4, wherein the predetermined speed is 80 km/h.
 6. Theelectromotive drive system of claim 4, wherein the predetermined speedis equal to or less than 50% of a maximum speed of the engine-drivenvehicle.
 7. An electronic controller of an electromotive drive systemfor an engine-driven vehicle, the electromotive drive system comprising:an electricity driven motor that, at a point in time, either operates ina drive mode or a power generation mode, wherein, in the drive mode, theelectricity driven motor drives the vehicle, and in the power generationmode, the electricity driven motor generates electric power, a batterythat stores electrical energy that is used to rotate the electricitydriven motor, an inverter that transforms electrical energy that isoutput from the battery into an alternating current to supply thealternating current to the electricity driven motor, and transformselectric power regenerated by the electricity driven motor into a directcurrent to supply the direct current back to the battery, a converterthat transmits, to a drive shaft of the vehicle, a rotation of theelectricity driven motor at a first conversion ratio independently of asecond conversion ratio at which a gas powered engine is driven, thefirst conversion ration being predetermined, and the electroniccontroller that controls an operation of the inverter, sets a firstmaximum driving force of the electricity driven motor, the first maximumdriving force being a maximum driving force of the electricity drivenmotor when the electricity driven motor is operating in the drive mode,and the first maximum driving force being in an absolute value, and setsa second maximum driving force of the electricity driven motor, thesecond maximum driving force being a maximum driving force of theelectricity driven motor when the electricity driven motor is operatingin the power generation mode, and the second maximum driving force beingin an absolute value, wherein the first maximum driving force beingsmaller than the second maximum driving force.
 8. The electromotivedrive system of claim 1, wherein the converter comprises a differentialgear.
 9. The electronic controller of claim 7, wherein the convertercomprises a differential gear.
 10. The electromotive drive system ofclaim 1, wherein the converter comprises a transmission.
 11. Theelectronic controller of claim 7, wherein the converter comprises atransmission.